Ousteri Lake (French: Lac Oustéri) is a lake shared between the union territory of Puducherry, and the state of Tamil Nadu. This lake is also called as Ossudu Lake as it is located in the village named Ossudu.

The lake extends to about 800 hectares in which 390 hectares lies in Puducherry and the rest in Tamil Nadu. It is the largest lake of Puducherry.

Ousteri Lake is a man-made lake, which is considered as one of the important wetlands in Asia. Different species of migratory birds reside in this lake throughout the year, for which the lake gains significance from the IBA (Important Bird Areas). In 2008 Ousteri wetlands in Puducherry was declared as a bird sanctuary-Oussudu Bird Sanctuary, while the wetlands in Tamil Nadu was declared in 2015.

The lake is one of the prominent tourist spot in Puducherry. The Puducherry Tourism Department Corporation has made a small boat club on the lake for tourists. The Puducherry government is proposing to make Ousteri as a National park. It also has proposed to set up telescopes to watch the birds and make the region as an eco-tourism destination.